Axial piston machine having an adjustable bent axis valve segment and a variable unit for the electrically proportional adjustment of the displacement

ABSTRACT

The invention relates to an axial piston machine having an bent axis segment which is adjustable by means of an actuating piston, and a variable unit for the electrically proportional adjustment of the displacement. The variable unit has a control piston for controlling the oil pressure which displaces the actuating piston, an electrically controllable proportional solenoid which acts on the control piston, a feedback spring on the control piston for retracting the instantaneous Bent axis valve segment and a adjustment mechanism for setting the start current. To set the start current, the adjustment mechanism has a setting spring which acts on the armature of the proportional solenoid with an adjustable force.

BACKGROUND OF THE INVENTION

The invention relates to an axial piston machine having an adjustableBent axis valve segment and a variable unit f or the electricallyproportional adjustment of the displacement according to the features ofclaim 1.

To set the displacement, axial piston machines of this type which areembodied as variable displacement motors have a Bent axis valve segment,the position of which can be adjusted by means of an actuating piston.The actuating piston is controlled by a control spool which, whendisplaced along its longitudinal axis, connects or disconnects ducts insuch a way that oil which is at a certain pressure is made available todisplace the actuating piston. Axial piston machines are often equippedwith electrically proportional adjustment means. The input signal in avariable unit of this type is an electric current which controls aproportional solenoid which displaces and acts on the control piston.The output signal is the hydraulic pressure which acts on the actuatingpiston.

In order to adjust the axial piston machine to the displacement set bythe adjustment device, a feedback spring is provided which closes thecontrol loop. The instantaneous swivel angle of the valve segment is fedback to the control piston by the feedback spring in such a way thatwhen the set value is reached, said control piston is returned to itsinitial position by the force of the feedback spring which opposes thesetting force of the proportional solenoid.

When starting up using hydrostatic drives, the variable displacementpump which supplies the motor is generally initially set to the maximumfeed volume, before the variable displacement motor, which is normallyinitially at the maximum angle when starting up occurs, is to beginswiveling. The electrically controlled, servo-adjusted hydrostaticvariable displacement motors are intended to depart from the initialswivel angle, and thus vary the displacement per revolution, at adefined current, the start current. Normally, a fixed current value isprovided by the electronics for this purpose.

However, the elements used for adjustment and closed-loop positioncontrol such as a proportional solenoid, piston, feedback springs etc.have unavoidable tolerances which leads to considerable fluctuations inthe forces which occur. The deviations in the force ratios are normallyso large that, without compensation for the tolerances, the variabledisplacement motors would actually depart from the initial swivel angleat very different start currents, and the driving behaviour wouldcorrespondingly vary from vehicle to vehicle.

The tolerances are normally compensated for by means of the feedbackspring system, the force action of which is directed counter to theactuating force of the magnet, with the result that the return forcewhich forces the system into the initial position is set. A complexadjustment mechanism and at least one further spring, the force actionof which is adjustable and is likewise directed counter to theproportional solenoid, are necessary for this purpose.

SUMMARY OF THE INVENTION

With the present invention, it is intended to produce an axial pistonmachine having an improved setting capability to compensate fordeviations from the desired start current.

According to the invention, this goal is achieved with an axial pistonmachine having an adjustable Bent axis valve segment which is adjustableby means of an actuating piston, and a variable unit for theelectrically proportional adjustment of the displacement, which machinehas a control piston for controlling the oil pressure which moves theactuating piston, an electrically controllable proportional solenoidwhich acts on the control piston, a feedback spring on the controlpiston for retracting the instantaneous Bent axis valve segment and aadjustment mechanism for setting the start current, the adjustmentmechanism having a setting spring which acts on the armature of theproportional solenoid with an adjustable force.

One particular advantage of the setting spring according to theinvention is the fact that the force of said setting spring is directedcounter to the force of the feedback spring, resulting in a particularlyfavourable setting capability for the force ratio between theproportional solenoids and the feedback spring.

The setting spring is preferably integrated into the proportionalsolenoid. The adjustment mechanism forms, together with the magnet, onestructural unit, which considerably reduces the number of individualelements for assembly.

A further advantage of the arrangement according to the invention isthat the setting spring and the armature of the proportional solenoidand also the control valve can be arranged along a common axis, alongwhich the feedback spring and the actuating piston can also be arranged,resulting in an inline return system, which requires no additionalattachments to the end housing, along the Bent axis.

The feedback spring is preferably held at one side by a bore of theactuating piston, while resting at the other side on the control spoolvia a spring seat. The setting spring force which acts on the armatureis adjustable by means of a adjustment screw which is arranged in thelongitudinal direction of the Bent axis, as a result of which it ispossible to adjust the preload with which the spring acts on thearmature, and thus the force ratio between the actuating force of themagnet and the spring force of the feedback spring, and also, as aresult, a defined start current.

Further features and advantages are disclosed in the followingdescription of the exemplary embodiment presented with reference to FIG.1.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of an axial piston machine.

DETAILED DESCRIPTION OF THE INVENTION

The figure shows an axial piston machine 1 of bent axis design having avalve segment 14, the swivel angle of which is adjustable by means ofthe swivel mechanism 13. The swivel mechanism 13 is actuated by theactuating piston 10, the displacement of the motor being set inaccordance with the swivel angle of the valve segment 14. 011 pressureis applied to the actuating piston 10 via ducts formed in the valveblock 11 and in the end housing 12 and during this time, it executes alinear movement along the Bent axis 15.

For the adjustment of the actuating piston, the variable unit 2 has acontrol valve and a proportional solenoid 3 which are arranged onebehind the other along the common Bent axis 15. When displaced alongthis axis 15, the control spool 7 opens or closes the ducts in the valveblock 11 which lead to the actuating piston, and is operativelyconnected to the ram or the armature 5 of the proportional solenoid 3.

An adjustment mechanism 16 is integrated into the proportional solenoid3, which adjustment mechanism 16 serves to compensate for tolerances andcomprises a setting spring 4 and a adjustment screw 17 which are botharranged in the direction of the Bent axis 15. The adjustable force ofthe setting spring 4 acts here on the armature 5, which is situatedwithin the winding 6, of the proportional solenoid 3 with a force whichopposes that of the feedback spring 9. The opening through which theadjustment screw 17 leads out is sealed off by means of a sealingelement 18.

The feedback spring 9 is clamped between the spring seat 8 of thecontrol spool 7 and the actuating piston 10. It rests at one side on thespring seat 8 of the control spool 7 and at the other side on theactuating piston 10 where it is partially held by a bore in theactuating piston 10.

The arrangement operates as follows:

If a certain current is applied to the proportional solenoid 3, itsarmature 5 is displaced, and with it moves the control spool 7 whichthus moves out of its initial position until the actuating force of themagnet 3 is in equilibrium with the opposing force of the feedbackspring 9. At the same time, oil pressure is applied to the actuatingpiston 10 via the unblocked ducts. Said actuating piston 10 likewisemoves out of its neutral position and, by means of the swivel mechanism13, adjusts the swivel angle of the valve segment 14 and thus thedisplacement of the axial piston machine.

As the actuating piston 10 is displaced out of its initial position, theforce of the feedback spring 9 increases and from then on overcomes theactuating force of the magnet 3, and it retracts the control spool 7again until, when the set swivel angle is reached, said control spool 7reaches its initial position again in which the ducts leading to theactuating piston 10 are closed. The adjustment mechanism 16 which isintegrated into the proportional solenoid 3 is provided to compensatefor tolerance deviations. The preload of the setting spring is increasedor decreased by turning the adjustment screw 17. Its spring force acts,via the armature 5 of the proportional solenoid 3, on the control spool7 and changes the force equilibrium of the proportional solenoid 3 andof the feedback spring 9 which is connected to the control spool 7 bymeans of the spring seat 8. The force of the setting spring 4 isdirected here in the direction of the actuating force of the magnetcounter to the force of the feedback spring 9.

A simple adjustment mechanism for setting the start current is producedby means of this arrangement, the complex and costly apparatuses whichare otherwise provided for this purpose being dispensed with.

1. Axial piston machine having a Bent axis valve segment which isadjustable by means of an actuating piston, and a variable unit (2) forthe electrically proportional adjustment of the displacement, which hasa control spool (7) for controlling the oil pressure which displaces theactuating piston (10), an electrically controllable proportionalsolenoid (3) which acts on the control spool (7), a feedback spring (9)on the control spool (7) for retracting the instantaneous Bent axisvalve segment and a adjustment mechanism (16) for setting the startcurrent, the adjustment mechanism (16) having a setting spring (4) whichacts on the armature (5) of the proportional solenoid (3) with anadjustable force.
 2. Axial piston machine according to claim 1, in whichthe force of the setting spring (4) is directed counterto the force ofthe feedback spring (9)
 3. Axial piston machine according to claim 1, inwhich the setting spring (4) is integrated into the proportionalsolenoid (3).
 4. Axial piston machine according to claim 1, in which thesetting spring (4) and the armature (5) of the proportional solenoid (3)and also the control valve are arranged along a common axis.
 5. Axialpiston machine according to claim 1, in which the setting spring (4),the armature (5), the control spool (7), the feedback spring (9) and theactuating piston (10) are arranged along a common axis.
 6. Axial pistonmachine according to claim 1, in which the feedback spring (9) is heldat one side by a bore of the actuating piston (10) and rests at theother side on the control spool (7) via a spring seat (8)
 7. Axialpiston machine according to claim 1, in which the setting spring (4)force which acts on the armature (5) is adjustable by means of aadjustment screw (17) which is arranged in the longitudinal direction ofthe Bent axis.